Synchronizing Hydraulic Cylinders 4

[RM111]

Hi,
I have a project with two cylinders involved to move exactly same amount under varying pressure. We have used Flow divider (Gear type) to divide the flow. The application is,8" bore 8" stroke two standard cylinders will be moving full stroke in 2 seconds. But even at very slow speeds we observe the cylinders miss 1/2 to 1" to be out of sync. The air bleeding was done enough.
Any body can throw me some light from their experience? I thought gear type will give a perfect division, seems like not.
Is their any other method w/o changing type of cylinders?
Lot of thanks in advance for your valuable suggestions, including usage of sophisticated controls.
Regards

 

[byrdj]

I would think something is wrong with the divider. Most split flow with a 98% accuracy. Assuming full piston diameter you have the lead piston full stroke with 1.74 gallons with the 1" lagging only getting 1.54, thus only 87% accuracy from your divider.

just curious, how much different in load between the two pistons?

It might be internal bypass leakage in the pistons. Does your piston have an orfice controlled leakage across the piston. The custom pistons I'm famialr with have a orfice below piston (power side) to upper (drain) to provide a constaint vent. (like 1 gpm at 1600 psig)

 

[greye]

hi there regarding your cylinder synchronizing.

i have used gear dividers and valve dividers, cylinders will stay in to about 15% of perfect synchronization.
in designing systems the cylinders should bottom out against a positive stop in both directions, this i have found helps greatly, also ensure your load is identical on both cylinders. with geared flow dividers i have found the accuracy to be about 15%, so on a 20 inch stroke the cylinders could be as much as 3 inches out, if the load was unbalanced between them. you could use two piston pumps driven from same motor this would give you about 5% accuracy. you could try mechanically linking the cylinders,rack and pinion torque arm proportional valves depends on the accuracy you require not easy, but try bottoming out both ways.

regards.
Greye.

 

[RM111]

Thanks for my friends Byrdj and Greye for their valuable inputs.
I am using bothe cylinders to come from opposite directions from both ends as if in a double action press. Till a point the matrial is pressed in between two cylinders. We press material and till the middle mould is filled up the cylinders will have same pressure I suppose. The cylinders will continue going in but pressure may vary depending upon several factors of further filling . From both you gentlemen's comments may be the total leakages (could be uneven at uneven pressures) of dividers as well cylinders should be the reason I suppose. If I get 98% I should be ok. I will check for orifice issues but we ordered standrd cylinders.
Th cylinders bottom out at both ends or at least rear ends.
Even though it will give perfect sync, I cannot envision any mechanical linkages to hold around 75 tons plus it comes in opposite directions as Greye suggested, in our set up. Also two piston pumps should be a better choice at last and tune perfect for what we need. But the cost may double up for all the hardware.
Even if I could get later 98% is there anything better way than gear dividers and linkages, any one knows for this application?
Agains thanking both my friends ....
Raj

 

[byrdj]

I believe Greye implied that the mechanical linkage could be used to control a directional proportional valve (DPV).

The mechanical linkage would be fairly simple. One piston would be the Master and the other cylinder being controlled by the DPV. There would be some tuning and timing required.

A rough linkage arrangement would be as

As the master extends, the linkage would position the DPV off null and the slave would extend. When the slave extension equals the master, the linkage would position the DPV back to null.

This master slave could also be done with electronics and a servo valve.

 

[hydromech]

Hello

Rotary geared flow divider are generally 98%-99% efficient. That is at top speed. The faster you go the more efficient they are. Also more noisy! All flow divider have "slippage", this is the amount of oil lost between the gears and the housing. The longer the flow divider turns the more oil will be lost through slippage. The slippage will be worst on the part of the flow divider driving the cylinder with the highest load.

Thus to get the maximum efficiency from a flow divider, the loads must be equal and the flow divider must be sized to run as fast as possible.

You also get what you pay for. The cheaper flow dividers have machined parts that have large clearances, allowing more slippage. The better flow dividers have ground parts and have have clearnces in the region of less the 1 thousandth of and inch.

I have a marine application with a USCG specification of 98% for the cylinder synchronisation. The cylinders extend 1.2 mtrs in 40 seconds and have equal loads.

They stay synchronised at all times.

Flow dividers are clumsy but applied correctly they can provide a cost effective solution for a tricky problem.

'Hope this helps.

Regards

 

[kcj]

I have had good service with gear dividers.

If you go with feedback, I would put positions sensors (Temppsonics, or linear pots) either inside or alongside the cylinders and close the loop electronically. Then you would have accel, decell, synchronizing, or whatever you want. I would not try the mechainical linkages on feedback control as there is too much tinkering both initially, and in maintenance.
kcj

 

[Ray2]

I also have a need for synchronizing two cylinders with 100% accuracy. I came accross this website (

http://www.hwhcorp.com/ml21954.pdf)(page

20), that has a syncronizing cylinder. I don't understand how the syncronizing valve on each piston functions, though. They will not sell it to me, they also have some cool push chain (won't sell that to me either). So I have been experimenting with having one cylinder that pushes two cylinders at the same time, these two cylinders are connected in series with another cylinder, (viola) the last two cylinders have to work exactly together. But big problem exists, the two cylinders in series are in a closed loop, how do you bleed them and keep them full of oil (I think that is what their syncronizing valve does). That is where I am right now. What I am trying may not be practical, anyone know?
Thanks
Ray D

 

[danhelgerson]

The best success I have had in synchronizing two cylinders has been when I had the opportunity to run the two cylinders in series. The cylinders were sized so that the area around the rod of the larger cylinder was equal to the area of the piston of the smaller cylinder. The piston end of the first cylinder is connected to port A on the directional valve and the rod end of the second cylinder is connected to port B. The rod end of the first cylinder and the piston end of the second cylinder are connected to each other.

If it is impractical to get cylinders with exactly the correct bore to rod ratios, getting them close may be good enough because the cylinder positions will be repeatable.

 

[Ray2]

danhelgerson,
I like the idea, it is similar to what I had in mind, but simpler. Question, how do you keep oil in and air out of the closed loop where the piston end is connected directly to the rod end?
Thanks,
Ray D

 

[jrnsr]

In theory, a pair of double acting/double ended cylinders (area equal both ends of cylinders) offer a simple master/slave arrangement, but oil bypassing piston seals can allow drifting.